1. Field of the Invention
The present invention relates to lead-free acid-resistant glass composition, and glass paste comprised of the same.
2. Description of Related Art
In manufacturing plasma display panels (PDP), a white dielectric layer, or a white back (W/B), is formed on a surface of a glass substrate that functions as a back plate disposed at the opposite side of a watcher of television. And rib walls, or partition walls, are projected therefrom to form a plurality of discharge spaces. Although the rib substrate provided with the white dielectric layer and rib walls are conventionally manufactured in a sandblast method or a thick film screen printing method, recent rib substrates are manufactured in a chemical etching method, or a wet etching method, because it is easy to form them with a superiority in precision in their sizes and shapes, for instance, as disclosed in JP 2006-509341 A.
In the chemical etching method, for instance, a rib material layer having an even thickness to form rib walls is formed on a surface of the white dielectric layer as covering the whole surface by such as taping or paste applying; the rib material layer is burnt; then the rib material layer is capped with a planar resist layer 50 to form a predetermined pattern as shown in FIG. 1; the rib material layer is etched by acid such as nitric acid. This dissolves and removes portions 54 of the rib material layer 52 indicated by slants in FIG. 1 as illustrating the predetermined pattern of the resist layer 50, to obtain the rib walls having high precision in sizes and shapes in line with the precise pattern of the resist layer 50. FIG. 1 illustrates the white dielectric layer 56, the back plate 58 and address electrodes 60.
In the above manufacture of the rib substrate using the chemical etching method, almost perfect removal of the portions of the rib material layer 52 under opens of the resist layer 50 approximately without dissolution of the white dielectric layer 56 is required to protect the electrodes under the white dielectric layer 56 ensuring the rib walls with an even height. Accordingly, the rib material layer 52 is required to be appropriately dissolved in an acid used such as nitric acid, and at the same time the white dielectric layer 56 is required to be acid-resistant. The remarkable difference in speeds between the etchings for the rib material layer 52 and the white dielectric layer 56 is preferable.
On the other hand, the smallest possible difference in material characteristics such as the thermal expansion coefficient or dielectric constant is preferable because the white dielectric layer 56 and the rib walls fixed thereon are to be exposed to plasma in the same space, for instance, in the same discharge space of the PDP. It is difficult to satisfy these requirements at the same time because the remarkable difference in acid resistance for glass materials accompanies with the remarkable difference in the material characteristics such as the thermal expansion coefficient or dielectric constant. The difference in acid resistance is required to be within the range where minimal etching is possible, for the smallest possible difference in material characteristics, in the manufacture of the rib substrate using the chemical etching method. As a result, it is difficult to satisfy the requirements in both characteristics and precision. Lead-free glass that is expected to be a substitute for lead glass against recent environmental pollution by lead glass is difficult to be the material for precise etching because it is inferior in acid resistance to lead glass. And glass for the white dielectric layer 56 is required to have a softening point of not more than about 650° C., preferably not more than about 600° C., and a thermal expansion coefficient near to that of the back plate 58, so that the glass can be formed on the back plate 58, or a substrate, made from such as soda lime glass. These requirements limit one in choosing appropriate materials.
Various lead-free glass compositions against environmental pollution are suggested for various uses from before. JP 2006-509341 A principally relating to partition formation by chemical etching using lead glass, for instance, discloses possible lead-free glass composition for the white dielectric layer. It also discloses an example of lead-free glass for the rib walls comprising (in weight percent; hereinafter, “%” means “weight percent” if there is not another definition) 19% of ZnO, 6% of SiO2, 37% of B2O3 and 38% of Li2O+Na2O+K2O, and another example comprising 22% of ZnO, 21% of SiO2, 30% of B2O3, 5% of Al2O3+ZrO2, 21% of Li2O+Na2O+K2O and 1% of BaO.
JP 2990194 B discloses a high acid-resistant lead-free glass composition for a glaze to reform the surface of a cement product, comprising in molar fraction 35-60% of SiO2, 16-26% of B2O3, 6-8% of Li2O, 4-17% of R2O and 5-15% of RO (where, R2O is selected from Na2O and K2O, and RO is at least one selected from a group of MgO, CaO, ZnO, BaO and SrO). JP 2002-179435 A discloses a high acid-resistant lead-free glass composition for formation of a color layer printed on a planar glass for an automobile, comprising 32-55% of SiO2, 4-25% of B2O3, 1-30% of ZnO, 1-15% of Al2O3, 7-20% of Li2O+Na2O+K2O, 1-15% of TiO2+ZrO2+SnO2+CeO2, 0-5% of P2O5, 0-5% of CuO, 0-5% of La2O3 and 0-3% of F+Cl. JP 2000-302480 A discloses a lead-free glass composition also for an automobile, comprising 10-36% of SiO2, 56-75% of Bi2O3, 3-7% of TiO2, 3-5% of Li2O, 0-2% of Na2O and 0-3% of K2O.
JP 2005-015280 A discloses B2O3—ZnO—BaO series lead-free glass with a low softening point having 0.1-10% (oxide equivalent) of Sn for transparent dielectric at the front side. JP 2000-016834 A discloses low softening point glass comprising 60% of Bi2O3, 30% of B2O3 and 10% of SiO2 for the white dielectric layer where the rib walls are formed by sandblasting. JP 2003-257242 A discloses lead-free glass for thick layer resistance paste for circuit substrate, comprising not more than 1% of alkali metal, 10-30% of Bi2O3, 25-40% of SiO2, 30-40% of BaO, 5-7% of ZnO, 4-7% of Al2O3 and 0.01-8% of B2O3.
However, JP 2006-509341 A does not disclose any preferable lead-free glass to the white dielectric layer, only discloses lead-free glass for the rib walls to be removed by chemical etching, and it includes a large amount of alkali oxide and Bi2O3 and is low acid-resistant and low water-resistant. The glass compositions in JP 2990194 B, JP 2002-179435 A and JP 2000-302480 A are utterly different from the white dielectric layer of the PDP in use as mentioned above, and the compositions are not appropriate for the white dielectric layer of the PDP because the burning temperature of the composition in JP 2990194 B is high and 700-850° C., and those concretely disclosed in JP 2002-179435 A and JP 2000-302480 A are comparatively high and not less than 650° C. JP 2005-015280 A discloses glass composition for forming a transparent dielectric layer without etching, and no mention on such as formation of rib walls by chemical etching or acid resistance of dielectric is provided. The glass composition in JP 2000-016834 A is used for the sandblast method, and is not appropriate for the white dielectric layer on which rib walls are formed by chemical etching. Furthermore, that composition is not appropriate in stability because it has remarkably little SiO2 that is the component of glass. The glass composition in JP 2003-257242 A is different from the white dielectric layer in use as mentioned above, and the burning temperature is high and not less than 700° C.
An example of high acid-resistant lead-free glass is borosilicate glass, for instance, PYREX® of Corning Incorporated, but it is not appropriate for the PDP because its softening point is high and about 820° C. Addition of alkali metal causes lowering of the softening point of borosilicate glass, but it also causes rising of the thermal expansion coefficient and lowering in acid resistance. Use of boric acid or phosphoric acid as the glass skeleton component is suggested to obtain glass having a low softening point, but these are remarkably low chemical resistant, and accordingly, it is not appropriate for the white dielectric layer for forming rib walls by chemical etching.
JP 2000-302480 A, JP 2000-016834 A and JP 2003-257242 A disclose Bi2O3 series glass including bismuth (Bi), a usual substitute for lead (Pb), being available and utilized in many fields. Since addition of Bi2O3 causes rising of the dielectric constant, regulation of the dielectric constant by addition of other material(s) is often required for electronics such as PDPs.
The white dielectric layer to have rib walls thereon formed by chemical etching is required to (1) be lead-free, (2) be acid-resistant, (3) have a low softening point, (4) have a thermal expansion coefficient near to that of a glass substrate and (5) have a low dielectric constant since the white dielectric layer covering address electrodes functions as their dielectric. Bi2O3 series glass preferably meets these requirements. However, it was found that cavities or cavity layers may be formed at an interface of the substrate 58 and the white dielectric layer 56 near an address electrode 60 made of such as thick layer silver, as shown in FIG. 1, upon formation of the white dielectric layer with Bi2O3 series glass significantly different in acid resistance from glass forming rib walls, for forming the rib walls by chemical etching. These cavities 62 cause disadvantages by raising the dielectric constant in the adjacency of the address electrode 60, lowering the discharge characteristics and, accordingly, causing unlighting in places.
It is therefore an object of the present invention to provide lead-free glass composition preferable for a white dielectric layer, being high acid-resistant, and having a low softening point, a thermal expansion coefficient almost equal to that of a glass substrate for the PDP and a low dielectric constant, and avoiding to have cavities formed, and glass paste comprised of the same.